The backlight device of a LCD is classified into two types: direct light and edge light (side light) as respectively shown in FIG. 1 and FIG. 2. In FIG. 1 where FIG. 1(a) is the cross sectional view of Fig. 1(b), a backlight device of direct light type comprises an array of light source 10, a reflective plate 12 and a dispersion plate 14 respectively installed in the front and back sides of light source 10. The backlight device of edge light type shown in FIG. 2 comprises a light source 20 seated beside light guide 22 and surrounded by a light reflector 24, a dispersion plate 26 and a reflective plate 28 respectively seated in the front and back sides of light guide 22. FIG. 2(a) is the cross sectional view of FIG. 2(b). In terms of brightness, weight, and power consumption, the backlight device of edge light type is classified into three types: single edge, double edge, and tri-edge. Shown in FIG. 2 is the single edge type, while FIGS. 3(a) and 3(b) where light guides 32 and 36 are respectively accompanied by double edge light source 30 and tri-edge light source 34, are respectively the double edge and tri-edge types.
For a backlight device of edge light type, the light guide is a plate made of Acrylic material (PMMA), to guide light from edge, so that the light is dispersed to form an even surface light source. The light guide is usually made of Acrylic particles processed by injection molding, or is an Acrylic plate.
To achieve an even brightness for light guide, material with refractive index different from that of light guide (acrylic plate in the example) is usually printed on the surface of the light guide, or refractive faces of different angles are usually directly formed on the surface of the light guide. FIGS. 4-6 show three popular types of embodiments wherein FIGS. 4(a), 5(a), and 6(a) are respectively the cross sectional views of FIGS. 4(b), 5(b), and 6(b). Shown in FIG. 4 is a light guide 22 with printed organic layer 38 comprising light dispersion particles (for dispersing light) formed on one surface thereof. Shown in FIG. 5 is a light guide 22 with coarse structure 40 made on one surface thereof to form a face comprising concavities and convexities. Shown in FIG. 6 is a light guide 22 with concavities dug on one surface thereon to form a face in the shape of sawteeth.
For a LCD of relatively large size such as those bigger than 12 inches in diagonal, an Acrylic plate is too heavy to be used as a light guide. Although the problem may be solved by changing the shape of the light guide, it is hardly achievable by injection molding to manufacture the light guide because of the limitation on the size and the thickness difference control in injection molding, thereby acrylic plate is currently still used as the light guide for mass production of LCD of bigger size.